1. Field of the Invention
The present invention relates to decorative laminates, to backing sheets which are useful in preparing such laminates, and to processes for making such backing sheets and such laminates.
2. Description of the Prior Art
Decorative laminates have been produced commercially in the United States and elsewhere for a number of years, and have found widespread acceptance as mar-resistant surfaces for wall-coverings, paneling, table tops, counter tops, vanities, and the like. These laminates are conventionally formed by consolidating several laminae under heat and pressure to form a unitary structure which usually carries on at least one of its surfaces a surface decoration which can range from something as simple as a solid color to something as complex as an embossed, simulated woodgrain finish.
Although the methods of preparing such laminates and the number and types of laminae can vary widely, the procedure generally involves the use of one or more sheets of corestock, depending primarily on the ultimate thickness desired, in combination with a decorative or print sheet and, if desired, a top sheet or overlay. The corestock, if corestock is employed, usually comprises an unbleached kraft paper which has been impregnated with a relatively inexpensive thermosetting resin such as phenolic resin, a polyester resin, and the like, which is easily cross-linked upon the application of the consolidation pressure and heat.
The decorative or print sheet usually has more stringent requirements than the corestock, in that it is usually pigmented, yet still must be capable of also being impregnated with a "noble" thermosetting resin (i.e. a resin which is also cross-linkable upon the application of the consolidation pressure and heat, but which exhibits little or no color deterioration upon the application of such pressure and heat and which prevents any strike-through or "bleeding" of the thermosetting resin used in the corestock. Two of the most common "noble" thermosetting resins used to impregnate the decorative or print sheet employed in the preparation of such laminates are urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates, although other resins such as polyester resins have also been employed as "noble" resins for the impregnation of such sheets). In many instances the decorative or print sheet must also be capable of being printed with a design such as a geometric pattern, simulated wood grain, etc., which survives the consolidation step virtually intact.
In a number of instances, and particularly when the decorative sheet carries a printed design, an overlay sheet is also incorporated as one of the laminae of the decorative composite. The overlay is generally a thin, high-quality, alpha-cellulose paper which is also impregnated with a "noble" thermosetting resin which is usually the same as that used to impregnate the decorative or print sheet being employed. The overlay sheet is usually designed so that it transparentizes completely during the consolidation of the laminae, thereby enabling the decoration and/or printing present on the decorative sheet to be easily seen in the finished laminate; yet it also imparts a greater degree of mar and abrasion resistance to the decorative laminate than would otherwise be obtained without such an overlay.
As indicated above, the various laminae employed to form the ultimate laminate are consolidated through the simultaneous application of pressure and heat. This is generally achieved by subjecting a sandwich of the various laminae to heated pressure platens until the desired consolidation has been accomplished. As would be expected, and in order to achieve manufacturing efficiencies, commercial laminating operations usually involve the pressing of more than a single laminate at one time, through the use of presses having multiple openings therein.
Although the configuration of such presses can vary widely, they generally comprise two heavy, heated platens on the top and bottom of the press, with additional heated platens in between, the number of which being dictated by the number of openings desired in the press in question. Each opening in any given press typically comprises the following elements in the following order: (a) a heated platen, (b) one or more sheets of caul stock, which are primarily intended to distribute pressure uniformly over the surface of the caul plate, (c) a metal caul plate, (d) the opening itself, (e) another metal caul plate, (f) one or more sheets of caul stock, and (g) another heated platen.
When more than one laminate is to be pressed in a given opening (i.e., a "book" or "pack" of laminates), the pair of laminae being pressed in such an opening are usually pressed in a back-to-back configuration. However, since the impregnated corestock material which is usually employed in preparing such laminates tends to adhere to the caul plate when only one laminate per opening is being consolidated in the press, or to another laminate composite in the stack when more than one laminate per opening is being consolidated in the press, it is also customary to employ a release sheet or liner between the corestock and the caul plate when only one laminate per opening is being pressed, or between laminates in a stack when a stack of more than one laminate is being pressed in an opening. Such liners or release sheets can be designed to release freely from the laminate or laminates after consolidation has occurred, but in most instances the release sheet or liner is designed so that it becomes an integral part of the consolidated laminate during the consolidation step. In those instances where a stack of laminates are being consolidated in a single opening of a press, at least one side of the release sheet or liner often carries a release coating, so that if it is intended that the sheet or liner become an integral part of one of the laminates in the stack being consolidated, this laminate with the release sheet or liner attached thereto can readily be separated after consolidation from the next adjacent laminate in the stack. These release sheets or liners are also often referred to as separator sheets, and are, for example, made of materials such as thin, high-density glassine.
After consolidation, and particularly in those applications where the resulting laminate is intended to be used in a structural application, rather than merely for its decorative effect, the laminate is ofttimes subsequently bonded to a substrate material such as plywood, hardboard, particle board, cement-asbestos board, and the like, to give it additional strength and rigidity for its intended structural use. Unfortunately, however, and particularly in those instances where conventional adhesives such as thermosetting adhesives, hot-melt adhesives, latex emulsions, animal glues, and the like, are employed, the bonding process usually requires, as the initial step, the sanding of the back surface of the laminate to ensure an adequate bond between the laminate and the substrate material to which it is being bonded. Not only does this sanding step, merely because it is an additional step, add to the manufacturing costs of laminated products produced in such a manner, but this step can also be responsible for increased waste of laminated material prior to its being glued to the appropriate substrate material. Since the laminates in question are relatively inflexible after the resins contained therein have been thermoset during the consolidation process, thin laminates (e.g., laminates comprising merely an overlay sheet, a decorative sheet, and single sheet of corestock or a single backing sheet) are particularly susceptible to damage during the sanding step due to their brittleness after consolidation. Thus, in addition to the increase in costs which are attributable directly to the sanding step added to the process, there is often an additional increase in costs which is attributable to an increase in the damage and waste. Although there are coating compositions in the prior art (c.f., for example, U.S. Re. Pat. No. 27,644) which are designed to provide a backing sheet which, after consolidation in a laminate, does not require sanding of the resulting laminate in order to achieve a consolidation laminate which is readily gluable to a reinforcing substrate, such laminates cannot easily be consolidated in contact with one another in a stack without adhering to one another during the consolidation process.